A trial run of a new SOx reduction additive preblended with catalyst in a full burn FCC unit yielded significant savings when compared to current costs of rare earth based SOx reduction additives
Maria Luisa Sargenti, Grace Davison
Many FCC operators use SOx re-duction additives to provide a fast, no-capital approach for controlling SOx emissions. For this objective, the low rare earth Super DESOX® additives from Grace Davison contain a high amount of cerium (Ce) to promote the oxidation reaction of SO2 to SO3, forming the metal SO4 on the additive surface. In an effort to minimize the impact of the global rare earth (RE) situation on the economics of SOx controlling additives, Grace is offering refiners the opportunity for a trial run with the Grace Davison SOX MCD. This newly developed additive has been designed to be a cost/effective solution to reducing SOx emissions in FCC flue gas.
The amount of RE in SOX MCD has been optimized and better distributed in order to preserve as much as possible Super DESOX ® performance. In this instance, an FCC unit described in Table 1 at a refinery in Western Europe benefited from an SOX MCD trial run. Since 2008, efforts had begun at this facility to seek SOx emissions control alternatives.
In the trial run, uncontrolled emissions were measured with continuous emissions monitoring systems (CEMS) gas analyzers. A base line correlation was selected for calculating uncontrolled SOx prior to Super DESOX® addition with the FCC catalyst noted in Table 1. In this trial, MgO in e-cat was monitored along with SOx emissions in the flue gas. Actually, the desired 30% SOx reduction was achieved with only 0.5 wt% Super DESOX®.
Super DESOX® has been used on a regular basis since 2009. For an easier operation the additive has
been supplied preblended with the catalyst. After the 2011 turnaround the refinery switched directly from Super DESOX® to SOX MCD. Since then, SOx control additive has been on a continuous usage. The same SOx base line (correlation) developed in 2008 has been used for the performance evaluation. At the current additive level, the SOX MCD is outperforming Super DESOX® (Figure 1).
Factors affecting additive performance must be accounted for (Table 2), including:
- Sulfur content in feed, which increases SOx available for removal
- Oxygen content in the regenerator dense phase, which drives oxidation of SO2 to SO3 (often the rate limiting step)
- Regenerator air distribution (good contact between additive and SOx improves kinetics)
- Catalyst circulation rate (performance is proportional to circulation rate)
- Stripping efficiency (improves regeneration of the SOx control additive)
- Regenerator temperature (Negatively affects the SO2 to SO3 reaction)
Cost Effective Alternative
In the past Super DESOX® has proven to be efficient in removing SOx under diverse operating conditions. A higher dosage of SOX MCD might be required to match the performance of Super DESOX®. The experience in full burn FCC regenerators is showing that 20-25% more SOX MCD is required. Nevertheless, after the commercial trials performed in today’s high rare earth cost environment, SOX MCD is the most cost effective alternative for SOx control in the FCCU.
Editor’s Note: This article was based on a presentation by Maria Luisa Sargenti (maria.sargenti@grace.com) at the Grace Davison Symposium in Munich, Germany (September 2011) and was based on data generated at European refinery up to July 30, 2011. At press time, the SOX MCD additive is available to refiners for further trials.
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